The present invention relates generally to forming polymeric coatings on substrates, and in certain embodiments to coating compositions, coated articles, and methods for their production, that involve the use of a blend of selected types of resins.
As further background, plastic substrates such as but not limited to polycarbonate, acrylic and polyester often are used in a variety of applications as replacements for more traditional materials such as glass. Plastics can offer excellent properties such as impact resistance and optical clarity while providing weight reduction and abundant design flexibility. Despite the numerous benefits that plastic offers, they have a major drawback in that they often do not provide the desired degree of chemical, scratch, mar and abrasion resistance, UV resistance and/or exterior durability.
It is known that both conventional thermal cured and ultraviolet (UV) cured coatings for exterior applications exhibiting scratch and abrasion resistance have offered adequate protection and performance of these various plastic substrates when direct exposure angles or durations are limited. It is also known that UV cured coatings are often the preferred coating of choice because they offer distinct advantages such as improved cycles times and reduced work in process over conventionally cured alternatives.
Despite the many advantages of current exterior coatings—and more specifically of UV curable coatings—they still do not meet the all expectations and often lack desirable properties and combinations of properties that are very difficult to achieve with traditionally available technology. Traditional UV topcoats for scratch and abrasion resistant applications most often are comprised primarily of high functionality and highly reactive acrylates. Although these characteristics impart the desired hardness properties, they are prone to cured resin shrinkage and steric hindrance. The steric hindrance results in a higher than desired number of unreacted acrylate bonds in the cured polymer resin which make the cured coating more susceptible to premature UV and environmental degradation. Additionally, known UV topcoats upon curing create a rigid, inflexible coating. Such inflexible coatings when subjected to conditions of expansion or contraction during processing or field use or when applied to materials containing high internal stress result in inadequate performance such as cracking, compromised exterior durability or other imperfections. Thus it can be difficult for a single coating composition to provide the desired level of a high surface hardness, scratch resistance, chemical resistance, UV resistance and exterior durability.
In preferred embodiments, the present invention addresses the need for UV curable coatings that along with high scratch and abrasion resistance offer properties of improved exterior durability, UV stability, thermal stability and flexibility especially for design applications that subject the coated material to strenuous environments. Additionally, the current invention describes the processes for using these materials and the resulting coated articles.